1. Field of the Invention
The present invention relates to a DC constant-current source, and in particular to a DC constant-current source capable of compensating for errors in the output current caused by changes in the output voltage of the DC power supply.
2. Description of the Related Art
Various types of circuits for constant-current source have been developed as needed. FIGS. 1 and 2 show circuits of first and second prior-art constant-current sources, respectively, which are of our present interest.
The circuit shown in FIG. 1 is provided with DC power supply 2, output-current setting circuit 13, current regulating circuit 14 made up of pnp transistor Q.sub.4 and resistor R.sub.4, a current-difference amplifier made up of pnp transistor Q.sub.8 and resistor R.sub.8, and constant-current output circuit 5.
Constant-current output circuit 5 (hereafter referred to as output circuit 5) is made up of a plurality of pnp transistors Q.sub.16, ---, Q.sub.n-1, Q.sub.n of the same characteristics with the bases interconnected through a base line and the emitters connected to the positive electrode of DC power supply 2 through emitter resistors R.sub.16, ---, R.sub.n-1, R.sub.n of the same resistance.
Output-current setting circuit 13, driven by DC power supply 2, generates a current signal I.sub.C2 (the collector current of transistor Q.sub.2). The current output of output circuit 5 is regulated to a value which corresponds to reference current I.sub.C2, as will be described below.
Circuit 13 includes a series circuit composed of resistor R.sub.3A, temperature-compensated npn transistor Q.sub.1 and constant-voltage source 1 connected in series between the positive and grounded negative electrodes of DC power supply 2. Constant-voltage source 1 supplies transistor Q.sub.1 with constant emitter potential V.sub.1 with respect to the ground potential. Transistor Q.sub.1 serves to provide base potential V.sub.B1 for biasing the base of transistor Q.sub.2, V.sub.B1 being V.sub.1 +V.sub.BE1 and V.sub.BE1 being the base-emitter voltage of transistor Q.sub.1. Resistor R.sub.3A is determined according to approximate equation R.sub.3A =(V.sub.2 -V.sub.1)/I.sub.3A, where V.sub.2 and I.sub.3A represent the output voltage of DC power supply 2 and a prescribed current which flows through Resistor R.sub.3A. Npn transistor Q.sub.9 supplies a fraction of its current output to transistor Q.sub.1 as base current I.sub.B1 so as to minimize any deviation of collector current I.sub.C1 of transistor Q.sub.1 from current I.sub.3A, i.e. to minimize base current I.sub.B9 =I.sub.3A -I.sub.C1 of transistor Q.sub.9. This allows the deviation to be regulated to I.sub.3A /(f.multidot.h.sub.FE1 .multidot.h.sub.FE9), an order of 10.sup.-4 .multidot.I.sub.3A, where h.sub.FE1 and h.sub.FE9 represent the current gains of transistor 1 and 9, respectively, and f denotes a fraction of the emitter current of transistor Q.sub.9 that is supplied to the base of transistor Q.sub.1.
Transistor Q.sub.2 has an emitter grounded through resistor R.sub.2 and is biased with the same base potential as that of transistor Q.sub.1. This causes the emitter potential of transistor Q.sub.2 to equal that of transistor Q.sub.1, provided that the difference in the base-emitter voltages of the two transistors, .DELTA.B.sub.BE, is ignored. As a result, the emitter current I.sub.E2 of transistor Q.sub.2, thus collector current I.sub.C2, becomes approximately V.sub.1 /R.sub.2. In this way, collector current I.sub.C2, which is an output of output-current setting current 13, is set to a desired value by adjusting resistor R.sub.2. Transistor Q.sub.2 is also temperature-compensated so that a change in collector current I.sub.C2 caused by a temperature change in transistor Q.sub.1 will be compensated for. The advantage of output-current setting circuit 13 is that it is capable of establishing a current of a given strength with a smallsized circuitry.
Transistor Q.sub.4 and emitter resistor R.sub.4 constitute an amplifier identical with each of the parallel amplifiers constituted by transistors Q.sub.16, Q.sub.17 ---, Q.sub.n and their emitter resistors R.sub.16, R.sub.17, ---, R.sub.n. The base of transistor Q.sub.4 is connected both to the bases of the group of transistors Q.sub.16, ---, Q.sub.n-1, Q.sub.n and to the collector of transistor Q.sub.4 by way of transistor Q.sub.8 to constitute a current-mirror circuit, wherein transistor Q.sub.4 is the input transistor and the group of transistors Q.sub.16, ---, Q.sub.n-1 and Q.sub.n are the output transistors. The collector of transistor Q.sub.4 is also connected to the collector of transistor Q.sub.2 through a branch point where difference current I.sub.B8 =I.sub.C2 -I.sub.C4, which corresponds to the deviation of collector current I.sub.C4 of transistor Q.sub.4 from collector current I.sub.C2, is branched off.
Transistor Q.sub.8, associated with resistor R.sub.8, provides a path of the base currents of the group of transistors Q.sub.16, ---, Q.sub.n-1, Q.sub.n and of transistor Q.sub.4. Transistor Q.sub.8 also acts to control emitter current I.sub.E4 of transistor Q.sub.4 so as to minimize difference current I.sub.B8 by the same operation as transistor 9.
When the output current of output circuit 5 decreases, base potential V.sub.BG of the group of transistors Q.sub.16, ---, Q.sub.n-1, Q.sub.n is raised. Since the base of transistor Q.sub.4 is voltage-biased by base potential V.sub.BG, the rise in base potential V.sub.BG causes a decrease in emitter current I.sub.E4 of transistor Q.sub.4, which results in an increase in base current I.sub.B8 of transistor Q.sub.8. Transistor Q.sub.8 acts to carry more collector current I.sub.C8, which causes base potential V.sub.BG to be lowered, whereby emitter current I.sub.E4 increases to minimize base current I.sub.B8, i.e. to minimize the deviation of I.sub.C4 from I.sub.C2. Since emitter current I.sub.E4 is an input of the currentmirror circuit, the increase in I.sub.E4 causes the output current of the current-mirror circuit, i.e. output current I.sub.o of output circuit 5. Thus, output current I.sub.o is regulated to the value corresponding to collector current I.sub.C2. In this way, collector current I.sub.C2 serves as a reference current to be referred to by collector current I.sub.C4.
Next, referring to FIG. 2, a second constant-current source of the prior art will be explained. The essential part of the constant-current source is identical with that of the first constant-current source shown in FIG. 1. The difference is in output-current setting circuit 10. In constant-current setting circuit 10, reference current I.sub.r is established by applying a constant voltage V.sub.1 across resistor R.sub.2 through negative feedback amplifier 11 of voltage gain 1 (a voltage follower) which serves as a buffer circuit. Reference current I.sub.r is determined from equation I.sub.r =V.sub.1 /R.sub.2, as is the case in the first constant-current source.
The operation of the circuit shown in FIG. 2 to stabilize output current I.sub.o is similar to that shown in FIG. 1.
A problem in the first constant-current source above has been that it is susceptible to changes in the output voltage of DC power supply 2. Let .DELTA.V.sub.2 be the change, and g.sub.m1, g.sub.m2 the transconductances of transistors Q.sub.1, Q.sub.2, respectively, then change .DELTA.I.sub.C2 in collector current I.sub.C2 caused by .DELTA.V.sub.2 becomes (.DELTA.V.sub.2 /R.sub.3A) (g.sub.m2 /g.sub.ml), which entails a change in output current I.sub.o of the constant-current source. Further, another problem has been that, while transistor Q.sub.1 and Q.sub.2 are temperature-compensated, output-current setting circuit 13 as a whole is susceptible to temperature changes.
A problem in the second constant-current source above has been that the buffer amplifier, i.e. negative feedback amplifier 11, requires a large size.